A twisted pair cable includes at least one pair of insulated conductors twisted about each other to form a two-conductor group. When more than one twisted pair group is bunched or cabled together, it is referred to as a multi-pair cable. In certain communications applications using a multi-pair cable, such as in high speed data transmission, problems are encountered if the signal transmitted in one twisted pair arrives at its destination at a different time than the signal transmitted at the same time by another twisted pair in the cable. In addition, when two or more wire pairs of different impedance are coupled together to form a transmission channel, part of any signal transmitted thereby will be reflected back to the point of attachment. Reflection due to impedance mismatch between twisted pairs bundled as a multi-pair cable results in undesired signal loss and unwanted transmission errors, greatly compromising the speed of data transmission.
To counteract electrical coupling (i.e. "crosstalk") between twisted pairs of wires bundled as a multi-pair cable, it is known to bundle the twisted pairs wherein each pair within the multi-pair cable requires a different distance, called a "twist lay length", to completely rotate about its central axis. Twist lay length also affects impedance, by affecting both the capacitance and inductance of the cable. Inductance is proportional to the distance between paired conductors taken along the lengths of the conductors, while capacitance in a cable is partially dependent upon the length of the cable. As may be appreciated, when a cable is constructed with small twist lay lengths to its twisted pairs, and the twist lay lengths differ from pair to pair within the multi-pair cable in order to minimize crosstalk, the changes in twist lay length from pair to pair are accompanied by large variations in the physical spacing between individual wires within the pair, thereby affecting inductance. Moreover, if every pair includes a different twist lay length, then the helical lengths of each pair of conductors vary widely, thereby affecting capacitance.
Impedance matching within a given multi-pair cable is critical to achieving high-speed data transmission. However, because the inductance and capacitance changes from pair to pair within a given multi-pair cable, a nominal characteristic or "averaged" impedance may be uncontrolled from pair to pair. In fact, within all cables heretofore known, there is a tendency for the averaged impedance of at least some pairs within a multi-pair cable, where the pairs all have small but different twist lay lengths, to be at or beyond an industry acceptable value.
Currently, the industry accepted value (based upon TIA/EIA 568A-1) for averaged impedance between twisted pairs is 100 ohms, plus or minus 15% (100.OMEGA..+-.15.OMEGA.). For example, in a four-pair multi-pair cable, each of the four pairs must have an average impedance within the industry-accepted values. Thus, impedance between pairs may vary by up to 30.OMEGA., or by about 27%.
As data transmission speeds have approached the gigabyte per second level, now achievable due to recent advances in various communications technologies, the variation between twisted pair averaged impedance within a multi-pair cable has been found to greatly affect data transmission performance. Therefore, current industry standards established for lower data transmission speeds are inadequate. Instead, at these required data flow levels, actual transmission speed is only achieved when averaged impedance variation is no less than 97.5.OMEGA. and no greater than 102.5.OMEGA. (100.OMEGA..+-.2.5.OMEGA.).
Thus, numerous attempts have been made within the industry to minimize differences between twisted pair averaged impedance within a multi-pair cable, at best by experimentally altering the insulation thickness. In one attempt, a cable is constructed having multiple twisted pairs divided into two groups of twisted pairs. The insulation thickness of the two groups is empirically optimized to a set value within each group of twisted pairs, and each twisted pair has a different twist lay length. However, even a minor modification often requires extensive and time-consuming additional experimentation to find an acceptable cable construction to accommodate the modification.
In another attempt to minimize averaged impedance, the wires within a twisted pair are joined along their length, thereby limiting an average center-to-center distance between wires within a twisted pair along its length in an attempt to limit inductance effects. Other methods also attempt to modify a single physical property between the twisted pairs, including by modifying the chemical composition of the insulating material, providing special chemical additives to the insulating material, and by adjusting both insulation thickness and insulation density.